1. Technical Field
The present invention relates to an electro-optical device substrate, an electro-optical device, and an electronic apparatus.
2. Related Art
As an electro-optical device, there is known an electrophoretic display device in which an electrophoretic element having a liquid dispersion medium and electrophoretic particles is interposed between a pair of substrates. Since such a kind of electrophoretic display device has a memory property, application of a voltage is not normally necessary to maintain a display state and a voltage may be applied only during a period in which the display state is changed. However, the voltage has to be held for respective pixels forming display during a given period, and thus it is necessary for each of the pixels to include a storage capacitor. Therefore, electrophoretic display devices having the storage capacitor are disclosed in, for example, Japanese Patent No. 4275671 and JP-A-2005-346090. According to Japanese Patent No. 4275671 and JP-A-2005-346090, in the electrophoretic display device, a pixel voltage can be sufficiently held by adding an electrophoretic element capacitor formed between a pixel electrode and a common electrode and a storage capacitor electrically in parallel.
In the electrophoretic display device disclosed in Japanese Patent No. 4275671 and JP-A-2005-346090, the storage capacitor includes a capacitor lower electrode formed in the same layer as that of a scanning line, a gate insulation film, and a capacitor upper electrode formed in the same layer as that of a data line with the gate insulation film interposed therebetween. When such a configuration is used, the scanning line and the capacitor lower electrode are disposed in the same layer. Therefore, it is necessary to prevent short circuit between the scanning line and the capacitor lower electrode. However, it is difficult to increase the size of the pixel when it is intended to form the pixel with high precision. Therefore, when a large space is disposed to prevent the short circuit, the storage capacitor having a sufficient capacitance value may not be formed. On the contrary, when a sufficient space is disposed to form the scanning line and the capacitor lower electrode in order to obtain a sufficient capacitance value, it is difficult to realize the high precision pixel.
As other methods, there have been suggested a method of using a material with a high specific inductive capacity in a capacitor insulation film and a method of increasing the storage capacitance value by making the capacitor insulation film thin. In these methods, however, since the capacitor insulation film is formed of a gate insulation film, for example, leakage current of a thin film transistor (hereinafter, abbreviated as a TFT) may increase. Therefore, there may be an adverse influence on the characteristics of the TFT.
When a sufficient storage capacitance value is not obtained, a feed-through voltage is increased when the TFT is turned off. Moreover, a variation in the feed-through voltage caused due to the variation in the capacitance of the TFT may not be allowed. As a consequence, display unevenness may be considerably shown particularly in the display of a half tone. The display unevenness is a particular problem of the electrophoretic display device which is a direct current display device and is a problem which occurs since the feed-through voltage is more irregular than that of a liquid crystal display device of an effective value response.
Moreover, when the sufficient storage capacitance value is not obtained, it is necessary to write the storage capacitance across a plurality of frames in order to refresh the pixel voltage. However, since the energy consumption (power×time) of the electrophoretic display device is mainly an energy consumed to charge a parasitic capacitance of the data lines, a problem may arise in that the energy corresponding to the energy used to charge the data lines may be unnecessarily consumed and the power consumption thus increases when the plurality of frames is driven.
For example, in the electrophoretic display device disclosed in JP-A-2005-346090, a gate insulation film which is a capacitor insulation film is etched to form a thin film. In this case, however, since not only a film forming variation of the gate insulation film occurs but also an etching variation occurs, regularity of the storage capacitors may deteriorate and a problem with display unevenness may thus occur due to a variation in the above-described feed-through voltage.
Here, the electrophoretic display device has been described as a prominent example, but such problems occur not only in the electrophoretic display device but also in other electro-optical devices which drive an electro-optical material using a pixel switching element such as a TFT and a storage capacitor.